1. Field of the Invention
The invention relates to a driving force control system for a vehicle, which controls driving force of the vehicle by changing the speed ratio of an automatic transmission, while the vehicle is traveling while being decelerated.
2. Description of Related Art
In Japanese Patent Application Publication No. 2007-170444 (JP 2007-170444 A), a driving force control system for a vehicle is described which performs control for inhibiting upshift when the accelerator pedal of the vehicle is rapidly released, and performs deceleration assist control including control for performing downshift when a brake is suddenly applied to the vehicle. The driving force control system described in JP 2007-170444 A identified above is configured to determine conditions of the deceleration assist control as described above, based on a traveling environment and traveling conditions of the vehicle. For example, the system determines whether the deceleration assist control is to be performed, and a control level used when the deceleration assist control is performed, according to a distance between the own vehicle and a vehicle in front, a road surface gradient, and/or the driving tendency of the driver, for example. In a control example described in JP 2007-170444 A identified above, when the driving tendency of the driver is sporty traveling tendency, the automatic transmission can be shifted down to a lower gear position during execution of the deceleration assist control. The sporty traveling tendency is a driving tendency with which a greater value or emphasis is placed on the power performance of the vehicle, and a quick response of the vehicle to driving operation is required.
In Japanese Patent Application Publication No. 2003-211999 (JP 2003-211999 A), a traveling control system that lets the own vehicle to automatically follow a leading vehicle is described. In the traveling control system described in JP 2003-211999 A, traveling data indicating the behavior of the vehicle, traveling environment, driving operation, etc. is sampled, during traveling of the vehicle operated by the driver, and the driving tendency of the driver (multiple regression coefficient) is obtained by making a multiple regression analysis on the traveling data. Then, the system is configured to let the own vehicle automatically follow the leading vehicle, by setting a target acceleration/deceleration based on the driving tendency.
Also, a driving force control system described in Japanese Patent Application Publication No. 2002-139135 (JP 2002-139135 A) is configured to calculate a recommended speed ratio based on traveling environment of a road, calculate the optimum speed ratio, based on the recommended speed ratio, driver's intention to change the speed ratio, and the actual speed ratio, and change the optimum speed ratio at a rate of change determined by a difference between the recommended speed ratio and the actual speed ratio.
As described above, in the control system described in JP 2007-170444 A, the driving tendency of the driver is estimated during traveling of the vehicle. Then, when the driving tendency is the sporty driving tendency, the automatic transmission is shifted down to a lower gear position, as compared with the case where the driving tendency is not the sporty driving tendency, when the vehicle is decelerated ahead of a corner or an intersection, for example. With the downshift thus performed during deceleration, the acceleration performance can be improved when the vehicle is re-accelerated after being braked.
In the meantime, the control system described in JP 2007-170444 A uniformly sets the gear position during deceleration at a point ahead of a corner or an intersection, depending on whether the driving tendency of the driver is the sporty driving tendency, and performs downshift to establish the thus set gear position. Also, the driving tendency, which is an estimated value, inevitably includes individual differences or estimation errors. Therefore, the gear position or speed ratio established after the downshift may not be adequate or appropriate. For example, if the downshift is not sufficient, further downshift may be performed when the driver depresses the accelerator pedal so as to accelerate the vehicle after turning the corner. Namely, the driving force actually generated may be insufficient, relative to the required driving force intended by the driver. As a result, the driver may feel strange or uncomfortable, or may feel that the acceleration performance or acceleration feeling is not good.